Integrated circuit (IC) manufacturers produce die on substrates referred to as wafers. A wafer may contain hundreds of individual die which are often rectangular or square in shape.
Die on a wafer, or unsingulated die, are tested to determine good from bad, e.g., defective or nonfunctional, before the die are singulated and packaged. The earlier a defective die is detected, the fewer subsequent processing steps are performed on the defective die, which results in a reduction of costs associated with individual wafer processing. For instance, often only good die are singulated and packaged into ICs. The cost of packaging die is expensive and therefore the packaging of bad die into ICs increases the manufacturing cost of the IC vendor and can result in a higher cost to the consumer.
Therefore, it is beneficial in semiconductor processing to detect and screen out defective die as early as possible in the manufacturing process. The defects may be introduced at various levels of production. For example, some defects are manifest immediately, while other defects are manifest only after the die has been operated for some period of time.
Reliability curves such as that shown in FIG. 1 can be used to express a hazard rate or die failure rate f(t) over time t, and often have a “bath tub” shape. The reliability curve illustrated in FIG. 1 may be divided into three regions as shown: (1) an infant mortality region, (2) a random failures region, and (3) a wearout region.
The infant mortality region begins at time t0, which occurs upon completion of the manufacturing process and an initial electrical test. Some die, of course, fail the initial electrical test. Inherent manufacturing defects are generally expected in a small percentage of die, even though the die are functional at time t0.
The relatively flat, bottom portion of the bathtub curve, referred to as the random failure region, represents stable field-failure rates which occur after the die failures due to infant mortalities have been removed and before wearout occurs. Eventually, as wearout occurs, the failure rate of the die begins to increase rapidly.
To discover those circuits that are susceptible to infant mortality, manufacturing processes have included high temperature testing of die for extended periods of time before shipping products to a customer. Such testing, known as “burn-in,” refers to the process of accelerating failures that occur during the infant mortality phase of component life in order to remove the inherently weaker die. Burn-in can occur before or after a die is packaged. Testing of unsingulated die, e.g., die not individually separated from the wafer, can be referred to as wafer-level burn-in (WLB) or wafer-level testing.
During wafer-level testing and/or burn-in, it can be beneficial to isolate defective die, e.g., shorted die, which may draw excessive current. The current drawn by the defective die can result in a reduced supply voltage level and/or current applied to functional die which may share the power supply. Such a reduced supply voltage level can result in reduced voltage uniformity across a wafer and may prevent functional die from being adequately or reliably tested.
Some wafer-level testing methods include using fuses associated with individual dies or groups of die to attempt to isolate defective die. In such methods, the fuse is blown if a die draws an excessive current. Other die isolation testing methods include using an external resistor, e.g., a resistor located off-die, to limit the current drawn by a die to a predetermined value. However, such methods may limit various testing modes by not allowing for multiple different current values used for various different testing modes. Examples of such testing modes may include native testing, built-in self test (BIST), built-in self stress (BISS), design for test (DFT), among other testing modes.